Bandwidth compression for television signals

ABSTRACT

A video signal received from a source (10) is bandwidth-compressed by filters (12, 14, 16), filter (14) being a temporal filter and filter (16) being a spatial filter. Selection of the filter to be used is dependent upon picture content. The transmitter reconstitutes in interpolators (44, 46) the signal which would be regenerated at the receiver, determines which filter gives the best results, and transmits an indication of which filter has been used in a digital signal associated with the analogue video signal. Preferably a determination of motion vectors associated with the signal is made and the digital signal indicates which of the determined motion vectors is applicable to different areas of the picture. By transmitting the control signal digitally with the analogue video signal the receiver circuitry is greatly simplified while its reliability is improved.

BACKGROUND OF THE INVENTION

This invention is concerned with bandwidth compression for televisionvideo signals.

If High Definition Television (HDTV) is to become a practicalproposition, then some method of transmitting it to the home isrequired. It is widely assumed that such transmissions will be viasatellite, since spare radio spectrum is not available elsewhere. Theprincipal problem in this case is the extremely wide bandwith of theoriginal HDTV signal--possibly in the region of 40 to 50 MHz for thecombined luminance and chrominance information. Such bandwidths can notbe accommodated in the 12GHz DBS (digital broadcast by Satellite) band.

The high transmission bandwidth required for HDTV will cause problemsnot only for terrestrial and satellite broadcasting but also for signaldissemination by other media such as videotape, videodisc and cable.Some form of bandwidth reduction is required in order to overcome thesedifficulties.

Methods of bandwidth reduction have been described which use sub-Nyquistsampling with ore-filtering in one, two and three dimensions. Morerecently motion adaptive pre-filtering techniques have been described,are for example NHK Laboratory Note No. 304, 1984, NINOMIYA et al "Asingle Channel HDTV Broadcast System" and a paper presented by FUJIO,SUGIMOTO and NINOMIYA at the 14th International Television Symposium,Montreux, June 1985, "HDTV Transmission Method (MUSE)". This system isbased upon the removal, by filtering, of image frequency components thatare assumed to be of little use to the eye. The filtered signal has amuch reduced bandwidth and can be re-sampled at a lower rate fortransmission.

The use of motion-adaptive spatial sub-sampling can yield impressivereductions in transmitted bandwidth. Still areas of the picture aretransmitted at full resolution but with the information beingdistributed over many different television fields. Moving areas aretransmitted at a reduced resolution, taking advantage of the eye'salleged inability to perceive detail in moving objects. This is theapproach used in the NHK's MUSE system (described in the above papers)and which has an objective bandwidth compression factor of 4:1, leadingto a final transmission bandwidth of only 8.1 MHz. This system has givenresults that, while in general encouraging, nevertheless can be variableand unpredictable. We have appreciated that the system performance willdiffer from receiver to receiver and also vary as reception conditions,and hence the carrier to noise ratio, changes, and that this arisesbecause the system detects which areas of the scene are moving and whichstationary at every receiver. Furthermore, the system is relativelyexpensive in requiring movement detection circuitry at every receiver.

SUMMARY OF THE INVENTION

The invention is defined in the claims below to which reference shouldnow be made.

In a preferred embodiment of the invention, described in more detailbelow, an analogue video signal to be bandwidth reduced is subjected totwo types of filtering in parallel, namely temporal filtering which isappropriate where stationary portions of the picture are concerned, andspatial filtering which is appropriate where moving portions of thepicture are involved. Which filter output is used at any instant isdependent upon the picture content, and the filtering control signal isnot only used at the transmitter/encoder but is also transmitted as adigital signal associated with the analogue video transmission signal tothe receiver/decoder.

The preferred system enables a highly detailed image to be transmittedin stationary and slowly moving areas of the picture. The remainingareas contain poorly correlated information such as revealed detail,shot changes and erratic motion. These areas are transmitted at a lowerdefinition since the human visual system genuinely requires less detailwhen image detail is poorly correlated from field to field.

The reduced bandwidth signal consists mainly of analogue sub-sampledpicture data, but also contains a digital conrol signal which we havefound to be crucial to the success of this type of system. The digitalsignal carries reliable information about the motion content of thetransmitted image and details of which coding method was used for eachmoving area.

The encoder and decoder are enabled to operate in precise unison withone another, so that the correct post-filter in the receiver/decoder ischosen area by area to restore the bandwidth reduced signal to adisplayable form. This avoids the need to try to estimate movement fromthe video signal received at the receiver, which is difficult to doreliably as the received signal contains only a quarter of theinformation available at the encoder and also includes a variable noiselevel. Also, the transmitted signal has, of course, been sub-sampled inan unknown manner. By transmitting the digital signal associated withthe analogue video signal, all decisions on the motion content of thescene are made at the encoder. The receiver is simply told how to decodethe signal. In this manner the receiver is simplified and at the sametime the overall system performance is improved by guaranteed trackingbetween encoder and decoder. In addition, display improvement techniquessuch as line and field rate up-conversion are simplified since reliablemotion information, previously lacking, is readily available from thedata channel.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described by way of example with reference to thedrawings, in which:

FIG. 1 is a block circuit diagram of a transmitter/encoder embodying theinvention:

FIG. 2 is a block circuit diagram of a receiver/decoder embodying theinvention for use with the transmitter/encoder of FIG. 1;

FIG. 3 is a block circuit diagram illustrating two ways of transmittinga digital information signal associated with an analogue video signal,(a) multiplexed onto the video signal and (b) in an accompanyingchannel; and

FIG. 4 illustrates a possible upgrade path from a system embodying theinvention using interlaced signals throughout from source to receiver.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The television system illustrated comprises an encoder shown in FIG. 1at the transmitter and decoders as shown in FIG. 2 at each receiver.

The signal from an HDTV source 10 is first converted in an ADC 11 todigital form for ease of processing and applied first to a diagonalspatial filter 12 to reduce diagonal components in the spatial contentof the signal. This provides a first 2:1 bandwidth reduction. After thisdiagonal filtering the signal is filter 14 is a vertical-temporal filterthe purpose of which is to reduce the signal bandwidth for stationaryand slowly-moving areas, and the second ore-filter 16 is avertical-horizontal filter or spatial filter to reduce the signalbandwidth for more rapidly moving areas. The two filters 14,16 may, infact, contain certain elements in common so that some of the circuitrycan be shared. Appropriate compensating delays 15 are included.

The filters can be based on the filters described in BBC ResearchDepartment Report No. 1975/36 "The filtering of luminance andchrominance signals to avoid cross-colour in a PAL colour system", Dr.J. O. Drewery, published 1975.

The reduced bandwidth signals are re-sampled at a lower rate inre-samplers 18,20 to form two alternative transmission signals. Theresampling uses a coarser sampling lattice which is sufficient tosupport the reduced bandwidth after filtering.

The outputs of the two resamplers 18,20 then form the video component ofthe transmitted signal and are applied to a combiner 21 and a channelencoder circuit 22. For the moment the circuit 21 can be considered as aselector switch which selects at any moment the output of eitherresampler 18 or resampler 20 in dependence upon a control input 24 whichindicates whether that portion of the signal represents stationary ormoving information respectively. The resulting video is converted toanalogue form in DAC 23 for transmission and is augmented by a digitalsignal which indicate the value of the control signal 24 in channelencoder 22.

At the receiver the received broadcast signal, after the usual RF and IFstages, is applied on an input 28 to a channel decoder 30. The channeldecoder 30 separates analogue and digital components of the receivedinput signal. The digital component is applied to a control blockgenerator 32 which reforms the digital control signal 24. The analoguecomponent is converted to digital form by ADC 31 before being applied tointerpolators 34 and 36 which reconstitute a displayable signal from thereceived signal as neither of the two sub-sampled signals is suitablefor direct display at the receiver. Interpolator 34 is a temporalinterpolator for use in stationary image areas and interpolator 36 is aspatial interpolator for use in moving areas. The interpolators restorethe resampled signal to the input sampling structure from the source 10.

A recombining circuit 38 is controlled by the control block generator32, and may be assumed for the moment to select either the output ofinterpolator 34 or the output of the interpolator 36 to apply throughDAC 39 to the receiver display 40. The recombining circuit 38 operatesin precise synchronism with the combiner 22 at the encoder, as itresponds to the same control input 24 due to the digital informationtransmitted as part of the signal.

The derivation of the control signal 24 will now be described. This isachieved in principle by restoring the resampled signals to adisplayable form in the encoder, using interpolators 44,46 identical tothe interpolators 34,36 in the decoder. Thus the encoder has access tothe signals that the decoder would generate if it were to use thecorrect type of interpolator. A subtractor 48 then subtracts the inputsignal from the output of the interpolator 44 and a subtractor 50subtracts the input signal from the output of the interpolator 46. Fromthe outputs of the subtractors 48,50, two respective error measurementcircuits 52,54 determine a measure of the "error" between the output ofthe associated interpolator and the input signal on s mean square ormean modulus basis. Thus the circuits 52,54 are able to compare as faras possible the signals which are appearing at the outputs of theinterpolators 34,36 at the decoder with the original source signal theyare supposed to represent (assuming of course a perfect transmissionchannel. An error processor or comparator 56 determines which of theseerrors is the more acceptable corresponding to greater fidelity in theresampled signal and generates the control signal 24 in dependence uponthat determination.

The encoder can be provided with two monitor displays 58,60 to enableviewing of the two interpolated signals but these serve no part in thecircuit operation.

In this approach to HDTV transmission, all coding decisions are taken atthe encoder where the original undistorted signal is available forcomparison. The transmission link is configured as an analogue imagesignal augmented by a rugged digital control signal; we have called thismethod Digitally Assisted Television (DATV). The decoder is simply toldhow to reconstruct a displayable signal rather than having to takedecisions itself, based on a transmitted signal with a much reducedinformation content and a much poorer signal-to-noise ratio. In thismanner, the system performance is enhanced at the same time as the costand complexity of the receiver are reduced.

Thus the system illustrated in FIGS. 1 and 2 is seen to be a hybridanalogue/digital coding method. The moving areas of the picture aredetected at the bandwidth-compression encoder of FIG. 1. The sub-sampledvideo is sent in analogue form and the signal from the encoderindicating whether movement is assumed to be present is sent, on anarea-by-area basis, in an associated digital signal. The combination ofthe analogue and associated digital parts of the overall transmissionstandard forming the two parts of the hybrid. The digital movementsignal, which can have a substantial capacity, is used to regenerate theoriginal HDTV standard at the receiver. The receivers are simplified,since they no longer need any motion detectors, and their performance ismade more predictable. At the same time the technique allows a betterfinal result to be achieved since, not only will the movement detectoroperate more satisfactorily when provided with the increased amount ofinformation available before subsampling, but the fact that it is sitedat the coder allows more sophisticated and expensive, but higherperformance, algorithms to be used. As shown, two regenerated versionsof the scene, one via the stationary mode and one via the moving mode,are compared, area by area, with the uncompressed original at the coder.If the stationary mode has the smaller mean squared error then thatparticular area Is deemed to be statIonary and vIce versa.

The prefilter 14 operates -n statIonary areas effectively to spread thetransmission over four fields for example with a quincunx pattern oneach field. A complete, highly detailed stationary image will thereforetake the same number of field periods to accumulate. In moving areaspriority must be given to smooth portrayal of motion. Thus bandwidthreduction must be achieved by pre-filter 16 by a further reduction inspatial resolution.

The prefilters, subsamplers and interpolators are not described infurther detail as they can, for example, be similar to those used in theNHK MUSE system described in the previously-mentioned papers.

The coding decision is taken on a block-by-block basis with each blockcomprising a certain number of pixels. The block size should becomparable to the aperture of the pre-filters and interpolators tominimise the appearance of artefacts at transitions from onetransmission mode to another, but not so large as to produce boundariesidentifiable from a normal viewing distance. A vertical and horizontaldimension of about 1/1OOth of a picture height proves to be a goodcompromise and yields manageable data rates, in the region of 1 to 2Mbits/sec, for the control channel.

The block is preferably diamond-shaped as this tends to make thediagonal junctions between blocks less visible. Thus a diamond-shapedblock of 8 by 8 pixels containing 32 pixels in all, is preferred, givingabout 55,000 blocks per picture. In any event, it is preferred to haveat least 1000 blocks per picture but for the blocks to contain in excessof 10 pixels and preferably over 25 pixels each.

The combiners 22,38 can operate as cross-filters rather than simpleselector switches to avoid sudden transitions which might be noticeable.

The source and display can operate to any of a range of desired higherdefinition standards, which need not be the same. The source standardwill depend upon the standard used in the HDTV studio. If it is assumedthat the source standard is 1125/60/2:1; i.e. 1125 lines per picture, 60fields per second, with 2 interlaced fields to each picture, then therewill be 1035 active lines, a horizontal bandwith of 30MHz (luminance)and an active line duration of 26μsec. Sampling theory indicates thatthis may be sampled at a rate of 1560 samples per active line, or 60MHz,without distortion. In practice some additional margin will be required;if this is about 10% then the number of samples per active lines risesto approximately 1700. If alternate-line transmission is assumed for thetwo colour difference channels, and if each colour difference signal hashalf the horizontal resolution of the luminance signal, then thecombined luminance and chrominance information becomes 2550 samples peractive line. If line and field blanking are omitted, the total videosampling rate is 1035×30×2550 or 79.2×10⁶ samples per second. If thiswere to be coded as an analogue signal without any sub-samplingbandwidth compression, a bandwidth of nearly 40 MHz would be required.

The required 4:1 bandwidth reduction can be achieved in two stages. Thefirst factor of 2:1 is realised by the diagonal filtering in filter 12.The second factor of 2:1 is achieved by temporal filtering in stationaryareas of the picture and by more severe diagonal filtering operating onthe moving parts of the picture. After this filtering process, thesignal is sub-sampled and can then be transmitted, in analogue form,with a bandwidth of only 1OMHz. This corresponds to approximately 80% ofthe capacity of a 27 MHz wide WARC channel, assuming that bandwidths ofup to 12 MHz may be sustained without causing excessive levels ofinterference.

If dual-channel digital sound is used, with a capacity of 1 Mbit/sec,and making an allowance for synchronisation codes, clamp intervals etc.some 2 Mbits/sec should be available for the auxiliary movement data.This would allow diamond-shaped blocks of say 4 field lines by 8 pixelsin the incoming HDTV signal to be coded with a one-bit signal toindicate whether the information in that block is moving or stationary(a bit rate of 1035×1700×30/32 or 1.65Mbit/sec) and still leave somespare capacity.

If 1920 samples per active line were used for the studio standard itwould be possible to convert to 1700 samples using multi-tapinterpolation, but alternatively the bandwidth compression could bebased on 1920 samples per active line and the number of chrominancesamples reduced to one third of the luminance rate i.e. 640 samples peractive line. The overall video bandwidth is then little affected. Thechoice of a higher luminance sampling frequency before transmissionwould help to reduce the loss of visible information in the diagonalfiltering stages.

The standard used at the source and display is not restricted by thepresent invention but can be any suitable higher definition standard. Inparticular the standards can be interlaced or sequential(non-interlaced) standards.

Turning now to FIG. 3, two ways in which the hybrid coding may beimplemented are shown. The first method shown at (a) is that used in thesystem of FIGS. 1 and 2. The incoming HDTV signal is applied to acircuit 100 for measuring the parameter in the picture content which isbeing used to select the appropriate bandwidth compression system, inthis case the circuit is effectively a movement detector. The HDTVsignal is then subjected to essentially analogue processing in aprocessor 102, including for example sequential to interlace conversion,and applied to a multiplexer 104. In practice this processing can beimplemented using digital techniques. The output of the parametermeasurement circuit 100 is subjected to any necessary digitalprocessing, such as block formatting to relate the parameter to thecorresponding area of the displayed picture, and then applied to anotherinput of the multiplexer 104 where it is combined with the analoguesignal for transmission over a single channel. Any conventional methodmay be used such as time division or frequency division multiplex. Themethod shown at (a) is most useful for broadcasting purposes.

However, as shown in (b) in FIG. 3 the multiplexer 104 can be omittedand the digital data signal transmitted over a separate channel. Thismight be more appropriate in situations such as videotape recording. Forexample, the analogue signal can be a standard interlaced signalrecorded in the normal way, and the data channel can be recorded on aconvenient audio channel, or even on a separate tape but locked to thefirst tape by a suitable time code.

For simplicity in this specification reference is made to "transmitting"the signal but it will be appreciated that this term includes otherforms of processing such as recording the signal as by using a recorder108 shown in FIG. 3(b).

The information as to which parts of the incoming picture are moving andwhich stationary is transmitted in the digital part of the hybrid signalin order to inform the receIver decoder how to correctly decode thebandwIdth-compressed HDTV signal. However, it can also be used at thereceiver for expansion.

Similarly, if high-frequency static detail makes heavy bandwidth demandson the analogue channel, the nature and location of these features canbe transmitted in the digital channel and used in the finalreconstruction. Features associated with a sequential scan can betransmitted in the digital channel to assist the reconstruction ofupconverted sequential display when the transmission has been interlacedto assist bandwidth conservation.

The hybrid method described is also helpful in an evolutionary approachwhere the television system is upgraded as technology improves. Thus alogical progression of HDTV standards can be foreseen as shown in FIG.4. Initially the source, transmission and display are all based on thesame, interlaced standard (shown in line 1 of the figure. However, alittle advance planning of the data channel would easily clear the wayfor the progressive introduction of more developed systems capable ofhigher resolution. At the same time, a second receiver, possibly with asmaller display, might not require the full resolution and could use thedata channel to control a much simplified decoder which treats all theincoming signal as if it were moving.

As receiver technology advances, the situation may proceed to that shownin line 2; the receiver upconverts the signal to a sequential scan fordisplay. If the transmission field rate is lower than 60Hz, it may alsobe necessary to increase this at the receiver in order to eliminatelarge-area flicker.

A further progression would be to allow parts of the studio signalprocessing chain to operate in a sequentially-scanned manner. Twoalternatives are shown in lines 3a and 3b of the figure; either thecamera is scanned sequentially, and the result converted to interlaced,or else a sequentially-scanned signal is generated from an interlacedsource, probably using sophisticated motion estimation methods. In theformer case, it is a relatively simple addition to allow an auxiliarydata signal to be carried along with the interlaced video; this datasignal will ease any subsequent conversion back to sequentially-scannedform and is the same in concept as the movement data carried by thetransmission system. Indeed the bandwidth compression coder for thetransmission path may well work better if allowed to use the datagenerated in the sequential-to-interlace conversion system; this flow ofinformation is indicated by the dotted line in FIG. 4.

The final progression would be to use sequential throughout the studio -this is shown in the bottom line of the figure. In this case theinformation in the digital channel would be primarily concerned withreducing the bandwidth demands associated with the large amplitudedynamic information and high frequency static information of theoriginal signal.

Various methods of display up-conversion are described in BBC ResearchDepartment Report No. 1983/8, "The Improved Display of 625-lineTelevision Pictures" by A. Roberts. Whilst some of the relevantinformation to assist receiver upconversion can be derived from theanalogue signal itself, the hybrid transmission allows essentialadditional information to be transmitted which could not be extractedfrom the analogue signal. Furthermore, there are some overall systemadvantages not associated with bandwidth compression which hybridtransmission provides. For example, the responsibility for the detectionof movement is transferred to the signal coder. Because fewer of thesewould be required, the cost and complexity can be substantially higher.Thus more sophisticated movement detectors can be used, having aproportionately better performance.

The system as outlined above and illustrated in FIGS. 1 and 2 suffersfrom two potential disadvantages. The first of these is that, even whenthe picture is perfectly stationary, the signal is still subjected tothe loss associated with the stationary-path diagonal filter. This canbe overcome by making this filter also motion-adaptive so that, for veryslow movement indeed, the diagonal filtering is removed completely: thepicture will then build up over eight fields. The additional movementinformation can be added to the auxiliary data channel; it may benecessary to increase the compression ratio of the sub-sampled videoslightly, or increase the size of the individual blocks into which thepicture is divided in the motion detector, in order to make theadditional bandwidth available. There is no need, though, to use thesame block size for the additional motion signal as for the main motionsignal. For example, the main motion signal can use blocks of 4 fieldlines by 8 pixels while the additional motion signal for slow speedsuses blocks of 8 lines and 16 pixels. This reduces the extra capacitythat must be accommodated by a factor of four (to 0.42Mbit/sec).

The second disadvantage of the system outlined above is potentially moreserious. It has been widely assumed in the past that the eye cannot makeuse of high spatial frequencies if they are moving, and this is indeedtrue for a fixed gazing point. For normal television pictures, however,the eye will generally attempt to follow moving areas of interest eitherby continuous motion for low speeds or by "saccadic" motion for highspeeds. That is, if the eye is following a moving object, then the imageon the retina is stationary and any loss of fine detail can becomenoticeable. The eye's spatial detail requirements in moving areas maywell be reduced but certainly not, in the case of uniform wellcorrelated motion, by the large factors assumed in most of theliterature.

This problem can also be overcome at the expense of requiring anincreased capacity in the auxiliary data channel. In this instance theamount of movement is estimated to produce a motion vector indicatingthe amount of motion any given area of the picture has suffered. Amethod is described in a paper presented at the 128th SMPTE TechnicalConference, Oct. 24-29, 1986 N.Y., Preprint No. 128-49, "HDTV BandwidthReduction by Adaptive Subsampling and Motion Compensation DATVTechniques", and in our British Patent Application 86 17320. In thismethod two successive pictures are correlated as a function ofdisplacement to determine one or more peak correlation valuescorresponding to respective motion vectors. The picture is then analysedin blocks of a few pixels square to determine which of the possiblevectors is most appropriate to that block. Other methods of estimatingmotion vectors are known from a paper presented to the 14thInternational Television Symposium, Montreux, June 1985 by SUGIMOTO,FUJIO and NINOMIYA, "Second-generation HDTV standards converter".

Such techniques can be used to measure the motion vectors of, say, thetwo most important moving items in the scene, one of which could be thebackground during camera pans, for example. These vectors can be sent tothe receiver once every field, requiring only a very small additionaldata capacity. If each picture block is then coded with two bits ratherthan one, four different conditions can be indicated--viz "stationary","moving with vector 1", "moving with vector 2" and "moving, but not withvector 1 or 2". The address fed to the field stores in the transmittercoder and receiver decoder is then adjusted to allow those areasdetected as moving with vector velocities 1 or 2 to be coded using thestationary algorithm and thus reproduced with the full static resolutionof which the system is capable. Strictly speaking this approach onlyworks when the motion velocities are integer values of picture elementsper field. However, interpolation can be used to cope with motionvelocities having a fractional pixel per field component. However,removing the major component of the scene motion aids the choice ofdiagonal filtering parameters and, in any case, the subjective effectsof sub-pixel movement errors may be sufficiently benign to make thisinterpolation unnecessary.

The way the system uses the motion vectors is that it displaces thereference frame of the high spatial detail pre-filter 16 in such a waythat moving areas appear to be stationary. They can then be transmittedin their correct spatial position and with full spatial detail.

The decoder is told the values of motion vectors for the following fieldduring vertical blanking, and is told which areas belong to which motionvector continuously via the control data channel. It then appliescomplementary displacements to its stationary interpolator's referenceframe to reconstruct highly detailed moving areas. Displacing pre-filterand interpolator reference frames rather than the bandwidth compressedsignal permits a receiver without motion compensation to use the motioncompensated signal, thereby preserving compatibility. In such areceiver, the motion compensated areas are decoded using a modifiedmotion mode, thus allowing them to be reproduced in the correct positionbut with the previously noted poorer spatial resolution.

The block sizes used for determining whether motion exists and what typeof motion it is do not, in fact, have to be the same.

It should be noted that both of these enhancements can be achieved in acompatible way, provided sufficient data capacity has been allocated atthe start of any broadcasting service. It is clear that receivers notequipped to deal with the additional slow movement information willmerely continue to diagonally filter all stationary pictures. In asimilar way, receivers not equipped to use the motion vector informationwill simply decode all areas of the picture not identified as"stationary" as if they were moving but with no measurable vector: inother words "moving with vector 1" and "moving with vector 2" willsimply be interpreted as "moving". This principle could, of course, beextended to include larger numbers of motion vectors in order totransmit scenes containing complex motion.

Although the arguments presented so far have been directed towards thetransmission of high definition images, the digital assistance principlecan be used to improve the performance of systems having a lower basicdefinition such as MAC or current PAL and NTSC. A simple motion signal,sent via a digital assistance channel, could make display up-conversionto a higher line number, or to a sequential scan, more effective byhelping to differentiate between motion and high frequency verticaldetail. The resulting potential increase in vertical detail could becomplemented in the horizontal direction by using bandwidth reductiontechniques, and here again a reliable motion signal would be valuable.

For example, with C-MAC, motion vectors with moving/stationaryindication can be transmitted on a block by block basis. Withteletext-type transmissions in PAL, motion vector information may beincluded to assist receiver motion detectors used for improved PALdecoding, or in up-conversion, for example.

The further addition of motion vector information could, with sufficientprocessing power, permit the interpolation of intermediate fields withcorrectly positioned moving objects. This would largely remove artefactssuch as the combing of moving horizontal detail caused by simple,non-motion corrected, temporal interpolation between adjacent movingfields.

A reliable motion signal could also simplify the implementation of videonoise reduction at the receiver by avoiding the need for the complexnoise measurement circuits required for a remote motion detector. Thiscould be a very useful facility for a satellite receiving system wherethe size of receiving dish is limited for other reasons.

We claim:
 1. A system for transmission of analogue video signals comprising, at a transmitter:an analogue video signal source; sampling means for sampling the analogue video signal generated by the source; bandwidth compression means for compressing the bandwidth of the video signal generated by the source selectively by two or more different methods in response to a control signal, and including sub-sampling means for sub-sampling the sampled analogue video signal generated by the source; means for converting the sub-sampled, bandwidth compressed video signal into analogue form; means for transmitting the bandwidth-compressed video signal in analogue form; control signal generating means responsive to the picture content of the video signal generated by the source to generate the control signal for successive picture areas to command the bandwidth compressing means to select a required one of the bandwidth compression methods for successive picture areas; and at a receiver;means for receiving the transmitted bandwidth-compressed video signal; means for reconstituting a signal for display from the bandwidth-reduced transmitted video signal selectively by two or more different methods in dependence upon picture content; and means for displaying the reconstituted signal; further including: means at the transmitter for transmitting in association with the analogue video signal a digital signal comprising the said control signal; and means at the receiver responsive to the transmitted digital signal to extract the control signal and command the reconstituting means to select the required one of the reconstituting methods for successive picture areas to reconstitute the signal for display.
 2. A system according to claim 1, in which the bandwidth compression means comprises a vertical-temporal filter and a spatial filter both coupled to receive a signal from the source.
 3. A system according to claim 2, including a diagonal filter between the source and the vertical-temporal and spatial filters.
 4. A system according to claim 3, in which the diagonal filter is adaptively dependent upon movement, and the digital signal transmitted to the receiver contains information relating to the mode of operation of the diagonal filter.
 5. A system according to claim 1, in which there are at least 1000 such areas in each picture.
 6. A system according to claim 1, in which each such area comprises at least 10 pixels.
 7. A system according to claim 1, in which the areas are diamond shaped.
 8. A system according to claim 1, including means for multiplexing the said digital transmitted signal with the analogue video transmitted signal.
 9. A system according to claim 1, in which the transmission comprises a recording operation.
 10. A system according to claim 9, including means for transmitting and recording the digital signal along with the associated analogue video signal.
 11. A system according to claim 1, including means for determining one or more motion vectors comprised in the picture and for transmitting in the digital transmitted signal the values of the motion vectors for that picture.
 12. A system according to claim 11, in which two or more labelled motion vectors can be transmitted, and the control signal is determined for successive picture areas as indicating that the picture is stationary, is moving with a defined one of the labelled motion vectors, or is moving in an undefined manner.
 13. A system according to claim 11, in which when motion is detected approximating to one of the motion vectors, the bandwidth compression means takes account of motion in the amount of the motion vector before determining whether temporal or spatial filtering shall be used.
 14. A system according to claim 1, in which the control signal generating means comprises reconstituting means like that at the receiver, and comparator means for comparing the operation of the reconstituting means in each of its methods of operation to determine the best method.
 15. A system according to claim 1, in which the receiver includes up-conversion means for converting from an interlaced transmitted video signal to a sequential display standard, the up-conversion means being responsive to the digital transmitted signal.
 16. A video signal transmitter, comprising:an analogue video signal source; sampling means for sampling the analogue video signal generated by the source; bandwidth compression means for compressing the bandwidth of the video signal generated by the source selectively by two or more different methods in response to a control signal, and including sub-sampling means for sub-sampling the sampled analogue video signal generated by the source; means for transmitting the bandwidth compressed video signal in analogue form; control signal generating means responsive to the picture content of the video signal generated by the source to generate the control signal for successive picture areas to command the bandwidth compression means to select a required one of the bandwidth compression methods for successive picture areas; and means for transmitting a digital signal comprising said control signal in association with the analogue video signal.
 17. A video signal receiver comprising:means for receiving an analog bandwidth-compressed video signal in which bandwidth compression of a sampled analog video signal is achieved selectively by two or more different methods which includes subsampling of the sampled analog video signal; means for reconstituting a signal for display from the received signal selectively by two or more different methods in dependence upon picture content; and means for displaying the reconstituted signal; including: means for receiving a digital signal transmitted in association with the analog video signal to extract a control signal therefrom and command the reconstituting means to select the required one of the reconstituting methods for successive picture areas to reconstitute the signal for display. 